Indigo liquid toner offset printing is a hybrid of electrophotographic printing and offset printing. Electrophotographic printing generally involves charging a photosensitive receptor drum, laser scanning an image or text area of the photosensitive receptor surface, depositing charged toner particles onto the image area of the photosensitive receptor drum to develop a latent image, transferring the toner image from the photosensitive receptor drum to a charged substrate like paper, and finally fusing the toner particles on the paper surface with heat and pressure. Offset printing simply means that there is an intermediate blanket cylinder that transfers the ink or toner image from its origin on the plate cylinder to the final substrate (i.e. paper).
In most electrophotographic printers, dry powder toner particles that consist of color pigment, charge director, and polymer binder are used. There is a lower limit of 7 to 9 microns on the particle size of these toners because as they become smaller, they can become airborne in the press. This results in printing problems and may cause health problems for the press operators. The large toner particles hurt print quality because of their inability to reproduce fine detail and acceptable colors. Therefore, high quality images cannot be printed with dry toner electrophotographic printers.
Liquid toner formulations contain toner particles as small as 1 to 2 microns because the solvent in the formula keeps them from becoming airborne. Images produced from these toners have higher quality due to their ability to be transferred precisely onto the photosensitive receptor surface. The solvent in liquid toner formulations must be removed either by evaporation prior to the transfer of toner to the paper surface or by drying after the toner has been transferred to the paper. In digital offset printing (e.g. HP Indigo printer), a thermal offset blanket cylinder is placed between the photosensitive image drum and the paper surface that is supported on the back side by an impression drum. The liquid toner is heated by the offset blanket cylinder to first cause the pigment-carrying particles in the liquid toner to melt and solvate into a uniform blend, and then to evaporate the solvent when the offset blanket cylinder transfers the liquid toner onto the paper surface. When the liquid toner contacts the cool paper surface, it becomes viscous enough to adhere to the paper and solidify immediately to allow fast printing. The thermal offset in an Indigo printer provides two advantages over dry powder electrophotographic printing: the paper does not need to be conductive in order to receive the toner and no fusion step is needed to melt the dry toner particles to the paper surface.
Toner adhesion failure has been a problem for both electrophotographic and digital offset printing. The toner adhesion failure is particularly severe for digital offset printing due to the lack of a fusion step to fix the liquid toner particles onto the paper surface. Poor toner adhesion results in incomplete transfer of the toner to the paper surface, leaving a residue behind on the offset blanket cylinder, and low resistance of the printed image to becoming dislodged in subsequent converting operations. A process called sapphire treatment is being used to coat the paper surface with a layer of polyethyleneimine to improve liquid toner adhesion. But sapphire treatment has drawbacks that include a limited paper shelf life of six to eight months, yellowing, the need to apply the coating in an additional step after papermaking, and a tendency for the toner image to scuff easily during finishing and handling.
The use of ethylene-acrylic acid copolymer to prepare suitable substrates for digital offset printing has been disclosed, for example, in EP 1273975 and 0789281. The polymers are typically dispersed in water prior to application to the substrate. However, the polymers are not dispersible in their acid form and must be at least partially neutralized in order to prepare a stable aqueous dispersion with low viscosity and suitable particle size.
Dispersions of ethylene-acrylic acid polymers have been prepared using amines such as ammonia and monoethanolamine, alkali metal hydroxides, alkaline earth metal hydroxides and mixtures of amines and strong bases. See U.S. Pat. Nos. 3,389,109; 3,872,039; 3,899,389; 4,181,566; 5,206,279; and 5,387,635. The preparation of an aqueous dispersion of polyethylene-co-acrylic acid polymer (20 percent acrylic acid by weight) in an aqueous solution of sodium hydroxide and ethanolamine is described in example 19 of U.S. Pat. No.5,387,635. However, the dispersion of the same polymer prepared in ethanolamine is not stable as evidenced by the observation that the viscosity of the dispersion increased dramatically overnight.
Accordingly, there is an ongoing need for improved paper surface treatments for digital offset printing applications using liquid toner formulations. These treatments should be stable, improve toner adhesion, have the ability to be applied during paper production, have good durability, and resist photoyellowing.